The use of stents as a medical corrective and preventive device is well known.
For example, U.S. Pat. No. 5,133,732 discloses that a stent can be implanted into a body vessel. The stent is a cylindrical body formed by a coiled generally continuous wire with a deformable zig-zag structure. The stent is further provided with means for preventing the stent body from stretching along its longitudinal axis. This stent is especially useful when implanting very long stents by means of balloon expansion. The entire content of U.S. Pat. No. 5,133,732 is hereby incorporated by reference.
In another example, U.S. Pat. No. 5,507,767 discloses that a self-expanding endovascular stent is formed of stainless steel wire which is bent into an elongated zigzag pattern. The zigzag pattern has a plurality of substantially straight wire sections of various lengths separating a plurality of bends. The zigzag pattern is helically wound about a central axis to define a tubular shape such that a majority of the bends is disposed in a helix. Adjacent bends in the helix are interconnected with a filament. The entire content of U.S. Pat. No. 5,507,767 is hereby incorporated by reference.
Magnetic resonance imaging (“MRI”) has been developed as an imaging technique adapted to obtain both images of anatomical features of human patients as well as some aspects of the functional activities and characteristics of biological tissue. These images have medical diagnostic value in determining the state of the health of the tissue examined. Unlike the situation with fluoroscopic imaging, a patient undergoing magnetic resonance imaging procedure may remain in the active imaging system for a significant amount of time, e.g. a half-hour or more, without suffering any adverse effects.
In an MRI process, a patient is typically aligned to place the portion of the patient's anatomy to be examined in the imaging volume of the MRI apparatus. Such an MRI apparatus typically comprises a primary electromagnet for supplying a constant magnetic field (B0) which, by convention, is along the z-axis and is substantially homogeneous over the imaging volume and secondary electromagnets that can provide linear magnetic field gradients along each of three principal Cartesian axes in space (generally x, y, and z, or x1, x2 and x3, respectively). The MRI apparatus also comprises one or more RF (radio frequency) coils which provide excitation and detection of the MRI induced signals in the patient's body.
The gradient fields are switched ON and OFF at different rates depending on the MRI scan sequence used. In some cases, this may result in a changing magnetic field on the order of dB/dt=50 T/s. The frequency that a gradient field may be turned ON can be between 200 Hz to about 300 kHz.
Uniformity in the static magnetic B0 field over the imaging volume is important for image clarity. When the field is not uniform, image distortions called “image artifacts” result. Additionally, if the gradient fields deviate significantly from their ideal linear character over the imaging volume, image artifacts develop.
Medical devices which are placed into a patient's body can cause the magnetic fields of the MRI system to deviate from their preferred characteristics for clear imaging. If a medical device comprises metallic components (such as iron), image artifacts result due to the metal's magnetic susceptibility properties distorting the MRI system applied magnetic fields. These are known as susceptibility artifacts. Additionally, if the medical device comprises conductive components, eddy currents develop in these conductive components when the MRI system's oscillating magnetic fields are applied. The eddy currents distort the net magnetic fields in the imaging volume, thereby providing another source for MR imaging artifacts.
After a stent is inserted into a patient, it is often desirable, over time, to determine if the stent is performing as expected. In the case, for example, of deploying a stent to correct a stenosis problem, it is desirable to determine if there is any indication of restenosis. This, as well as for other medical situations, requires obtaining images of the volume inside the stent. Due to the image artifact problems, described above, inherent in metallic, conductive stents, it is not possible to obtain clear MR images of the interior volume of the stents.
Attempts have been made to overcome these problems. The article, “Artifact-Free In-Stent Lumen Visualization by Standard Magnetic Resonance Angiography Using a New Metallic Magnetic Resonance Imaging Stent” by Arno Buecker, et al., Circulation, Apr. 16, 2002, pp. 1772-1775, discloses that a handmade stent can enhance the imaging ability of the stent's lumen. However, the handmade prototypes lacked a radial force comparable to standard stainless steel stents. Additionally, the article discloses the use of a contrasting agent to enhance visualization of the stent lumen.
The article “MR Imaging of Vascular Stents: Effects of Susceptibility, Flow, and Radiofrequency Eddy Currents” by Lambertus W. Bartels, et al., published in Journal of Vascular and Interventional Radiology, volume 12, Number 3, March 2001, pp. 365-371, describes the various image artifacts that prevent clear imaging of stent lumen.
The article “Improved Lumen Visualization in Metallic Vascular Implants by Reducing RF Artifacts” by Lambertus W. Bartels, et al., published in Magnetic Resonance in Medicine, 74:171-180 (2002), describes attempts at imaging metallic stents lumen by using contrast agents and by increasing the power deposited into the patient during the MRI procedure. The power deposited into the patient body, measured as the Specific Absorption Rate (SAR) can be harmful to the patient undergoing an MRI if set too high. In addition to the higher power deposited into the patient, the article discloses that adjustments in the image reconstruction process need to be implemented.
U.S. Patent Application Publication US 2002/0188345 A1, published Dec. 12, 2002, discloses an expandable metallic stent that has discontinuities of non-conducting material. These eliminate electrically conducting paths in the stent rings and cells. This makes the stent easier to image with magnetic resonance imaging (MRI). The entire content of U.S. Patent Application Publication US 2002/0188345 A1 is hereby incorporated by reference.
WIPO PCT publication WO 03/015662 A1 discloses that a metallic endoprosthesis causes no significant artifacts on images taken by magnetic resonance tomography (MRT), as a result of the combination of the production materials with a special design, which permits an evaluation of the externally adjacent region and the lumen of the endoprosthesis by means of MRT.
Although the above-described stents can be used to help a patient, these stents still cause magnetic resonance imaging artifacts. Moreover, the above-described stents prevent the imaging of the volume within the stent during magnetic resonance imaging.
Therefore, it is desirable to provide a stent which produces little to no magnetic resonance imaging artifacts. Moreover, it is desirable to provide a stent that allows the imaging of the volume within the stent. Furthermore, it is desirable to provide a stent which produces little to no magnetic resonance imaging artifacts and/or allows the imaging of the volume within the stent.